What Ben Franklin, Sweden, and a lucky break in the clouds have to do with the distance to the Sun.
In 1716, sixty-year old Sir Edmund Halley called on astronomers all over the world to leave their cozy observatories, travel to the edges of the known world, set up their telescopes, and turn their eyes toward the sunrise on the morning of June 6th, 1761, when the first Transit of Venus of the scientific age would march across the face of the sun.
In the eighteenth century, the solar system had a shape but not a size. By timing the entrance and the exit of Venus across the sun from latitudes all over the world, Halley explained, astronomers could roughly calculate the distance between the Earth and the Sun — a “celestial yardstick” for measuring the universe, as Andrea Wulf calls it in her excellent book Chasing Venus: The Race to Measure the Heavens.
It was the first worldwide scientific collaboration of its kind, a mathematical olympiad six hours in duration, with years of planning and seconds that counted. Today, more than 250 years after this grand experiment that required astronomers all over the world to gather together and look to the sky at the exact same moment, we will experience the last transit of our lifetime (unless modern medicine makes us survive to December 2117, when the next one will take place).
Astronomers had two chances to catch this rare event, one in 1761 and one eight years later, in 1769. (The first viewing had taken place in 1639, observed by two astronomers in the English countryside.) The first outing was a gamble that the venture could work, the second was a high-stakes race to get it right. Mild-mannered scientists would have to become swashbuckling adventurers.
In 1761, the best viewing locations for the transit were also the most remote. The best times to capture data, explained Halley, were the shortest and longest durations and required locations in the extreme north and south, and unified efforts of the British, French, Russian, and Swedish governments, despite the fact that the British and French were at war — an unlikely alliance as heartening as the Christmas Truce of 1914.
Months before the 1761 transit, a fleet of international astronomers was dispatched to South Africa, India, Siberia, Mauritius, eastern Finland, Newfoundland, and the remote island of St. Helena with instructions to set up their massive telescopes in some of the most inhospitable places on Earth. They required an impossible union of plentiful government funding, smooth sailing, open roads, understanding locals, accurate clocks, keen eyes, and clear skies.
In the morning of June 6, 1761, the Queen of Sweden hurried to the Stockholm observatory to watch as the boiling edge of Venus touched the edge of the nighttime sun at 3:21AM. A Swedish astronomer stationed in eastern Finland was prepared to time the entry at 4AM, but local farmers had decided that the morning was also a good time to set fire to unwanted brush, and the smoke obscured much of his precious view. India caught the entrance at 7AM, Jakarta at 9AM, but most had trouble securing both the entrance and the exit. Clouds plagued the viewings at Cape Town and St. Helena, but Harvard professor John Winthrop was able to carefully time Venus’ exit while stationed in Newfoundland, the only calculation in North America.
Once the data was collected, a second problem emerged: there was no standard measurement on Earth for a proper calculation. A minute in India would be different than one in Halifax which would be different than one in South Africa. The same for feet, inches, meters, miles. The task at hand was monumental, impossible, and essential for mapping the heavens. When the final figures were tallied for the distance of the Earth to the Sun, the range of answers covered over 20 million miles. It was a poor mathematical showing.
Eight years later, in 1769, the scientifically-minded Catherine the Great was in power in Russia, as was George III in England, and both were eager to spend heavily on a new army of astronomers. Over four hundred viewings were scheduled, including locations in Lapland and Baja California. The brightest minds of the Enlightenment rallied around the cause: Benjamin Franklin spearheaded the calculations in the colonies, and Captain James Cook shuttled a fleet of scientists and naturalists to Tahiti, where they viewed the transit with cloudless blue skies.
It took three years for Captain Cook to return to England with his essential data, and the calculations when completed narrowed the margin of error from 20 million miles to just 4 million. But the Venus expeditions had gathered not just numbers, but plants and animals and observations of customs from around the world. Chasing Venus chronicles a rare planetary event that happened at a rare juncture in human history, when the age of empire, the age of science, and the age of curiosity brought the world together for just a few moments — to achieve the measure of the universe.